“ECS is actually 6C when arctic sea ice feedbacks are taken into account”
you are sorely mistaken if you think that we wont experience <1,000,000 km^2 of arctic sea ice by June 21 under 2XCO2 forcing. This means an additional globally averaged albedo feedback of about 0.23 watts per meter squared for each 1,000,000 km^2 loss. (may be as high as 0.3 but I low-estimate) The 1970-1980 mean of ice for June 21 is about 11,000 KM^2 so this ice free state is equivalent to about 2.3 watts per meter squared globally averaged forcing feedbacks compared to pre-industrial.
Add that to the 3.7 of 2XCO2 and you get 6.0 Watts per meter squared without other feedbacks This calculation neglects the reduction of oceanic Dimethyl Sulfide production which is projected to yield (median) +0.3 C of warming feedback, as well as other feedbacks such as carbon cycle, frozen soils and far-infrared emissivity reductions and increased arctic algae blooms causing further albedo effects.
not to mention northern hemisphere snow-cover anomalies.
since our best estimate so far is 0.6C of globally averaged warming for each 1.0 Watt per meter squared that is 3.6C of warming just for CO2, WV/Lapse Rate and Arctic sea ice. Add the 0.3 DMS feedback, emissivity and algae bloom effects and you end up with ~4.3C as a baseline response.
Then you can add cloud feedbacks, snow cover albedo effects, frozen soil effects and carbon-cycle effects.
as you can see the 6.0C ECS is a low-end estimate. This is why we will have to engage in geoengineering activities, including dimming and CO2 atmospheric extraction.